124 TEXTBOOK OF BOTANY 



In plant cells the hydrolysis of sucrose is catalyzed by a specific 

 enzyme known as invertase ( sucrase, or saccharase ) . The reverse proc- 

 ess, the synthesis of sucrose from glucose and fructose in a plant, is also 

 accelerated by invertase or other enzymes. Many enzymes have the 

 property of catalyzing a reaction in either direction, the direction being 

 dependent upon other conditions in the cells. 



Invertase that has been extracted from plants may be obtained from 

 chemical supply companies. When sucrose is dissolved in water with a 

 small amount of this extracted invertase, it is hydrolyzed as indicated 

 above. If the solution is heated to 40-50'' C. the reaction is most rapid; 

 it decreases rapidly above this temperature. Temperatures of 60° C. or 

 above completely inactivate many enzymes when they are wet. Most of 

 them are destroyed at temperatures below the boiling point of water. 



The activity of enzymes is influenced by many conditions within the 

 cells. Among these may be mentioned various concentrations of acids, 

 alkalies, salts, the substances acted upon, and the substances formed. 



The potency of enzymes in accelerating chemical change in living 

 cells is apparently due to the ease with which they unite with certain 

 substances involved in the reaction and form unstable intermediate com- 

 pounds. Through reactions of these unstable compounds the enzyme is 

 again liberated, and only the stable products of the whole series of reac- 

 tions accumulate. For instance, sugar and free oxygen are the stable end 

 products of photosyntliesis, but between them and the initial raw ma- 

 terials (CO- and H^O) several temporary intermediate compounds are 

 formed. Chemically trained students in increasing numbers have tried to 

 discover what these intermediate compounds are and the order in which 

 they are formed. Some of the problems will be mentioned briefly. 



Intermediate compounds in photosynthesis. Formaldehyde has often 

 been proposed as the first intermediate substance in photosynthesis, but 

 alwavs with inadequate proof. Recent in\'estigators are giving more at- 

 tention to enzymes and the part that chlorophyll may play in the process. 

 For many years it has been known that chlorophyll absorbs certain ra)'s 

 of radiant energy. This is only another way of saying that chlorophyll 

 transfomis certain radiant energy to some other kind of energy— or 

 better, that certain photons of light upon striking some part of the 

 chlorophyll molecule impart their energy to it. As a result, chlorophyll 

 becomes chemically reactive, and in this state it probably unites with CO- 

 and H-O, forming an intermediate unstable compound. Merely by way 



